Feedback-ID: ia2694551:Fastmail
Date: Tue, 7 Apr 2026 11:22:18 +0200
From: Alvaro Herrera <alvherre@alvh.no-ip.org>
To: Antonin Houska <ah@cybertec.at>
Cc: Srinath Reddy Sadipiralla <srinath2133@gmail.com>,
	Tom Lane <tgl@sss.pgh.pa.us>, Andres Freund <andres@anarazel.de>,
	Noah Misch <noah@leadboat.com>, vignesh C <vignesh21@gmail.com>,
	Amit Kapila <amit.kapila16@gmail.com>,
 Mihail Nikalayeu <mihailnikalayeu@gmail.com>,
	Matthias van de Meent <boekewurm+postgres@gmail.com>,
 Pg Hackers <pgsql-hackers@lists.postgresql.org>,
	Robert Treat <rob@xzilla.net>
Subject: Re: Adding REPACK [concurrently]
Message-ID: <202604070916.5rj6kvjx3mrk@alvherre.pgsql>
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On 2026-Apr-07, Antonin Houska wrote:

> > @@ -1739,6 +1739,8 @@ SerializeSnapshot(Snapshot snapshot, char *start_address)
> >  
> >   Assert(snapshot->subxcnt >= 0);
> >  
> > + MemSet(&serialized_snapshot, 0, sizeof(SerializedSnapshotData));
> > +
> >   /* Copy all required fields */
> >   serialized_snapshot.xmin = snapshot->xmin;
> >   serialized_snapshot.xmax = snapshot->xmax;
> > 
> > thoughts?
> 
> Could you reproduce the failure in your environment?

Yeah, he did.

> I haven't thought of this explanation because BufFileWrite() only copies the
> data to a buffer in the BufFile structure and BufFileDumpBuffer() writes the
> buffer. Maybe valgrind is able to track the copying?

Yeah, apparently it keeps track of tainted bytes somehow.  Clever.

The change to palloc0() that I was proposing did not fix the problem,
because the stack allocated struct overwrote those zeroes with the
uninitialized padding bytes.

I ended up with an equivalent fix to Srinath's -- zero-initializing
the stack-allocated struct, so that the bytes that end up copied by
memcpy() are all defined.  Srinath confirmed that in his environment the
valgrind failure goes away, so I think we're good.

-- 
Álvaro Herrera        Breisgau, Deutschland  —  https://www.EnterpriseDB.com/
"No tengo por qué estar de acuerdo con lo que pienso"
                             (Carlos Caszeli)